I use Fusion 360 for my design work so I usually model a rectangular board with the same thickness and do a facing operation with the largest endmill I have. Stepover of around 50%. I’ve found climb cutting gives the best performance as conventional cutting will leave a fuzzed edge.

On thing to be sure of, your slab may be twisted as well as cupped. This will cause it to rock a bit as it’s being flattened… which means it doesn’t come out flat. What I do is use the tape/CA glue method to hold down the slab, and try to find my pivot points… that is… when I lay the slab on the table and push down on the corners, what part of the slab remains on the board? Tape and glue in that area first. The slab will still move and rock, so where there is movement, put in folded notepaper or some other wedge to brace the slab so it doesn’t rock. Just be sure you do it roughly evenly on opposite sides around the slab, and not just on one side. If you do just one side, you’ll end up with a wedge shaped board - thin on one end, thick on the other. When you flip it to flatten the other side, you’ll end up taken lots of material off one end just to get it down to the thickness you left at the other.